Japanese Patent Application Laid-Open No. HEI 6-310101 has conventionally been known as a technique in such a field. In the gas (deuterium) discharge tube disclosed in the publication mentioned above, two metal barriers are disposed on a discharge path between an anode and a cathode, whereas each metal barrier is formed with a small hole which narrows the discharge path. As a result, light having a high luminance can be obtained through the small holes on the discharge path. If three or more metal barriers are provided, a higher luminance is obtained. Light having a higher luminance is obtained as the small holes are made smaller.